In the Czochralski process for drawing or pulling single crystals from a seed crystal, a susceptor enclosure also known as a crucible support or holder, is often utilized as the surrounding wall of a rotary melt containing crucible which usually is provided with a quartz liner. Because of the high temperatures involved in the crystal pulling process, a slit or gap is provided in the susceptor wall which extends from end to end thereof to permit thermal expansion and contraction thereof. Typically, such susceptors may be eleven inches in diameter (outer) and have a 3/4 inch wall thickness, and the slit may be 1/4 inch in width. In practice, it has been found that such susceptors have a relatively short service life and must be replaced after three to four crystal pulling runs because the susceptor sleeves do not return to original shape upon cooling but tend to progressively expand with each run to the extent that they finally contact the heating element which surrounds them. This occurs if a typical graphite sleeve of this size warps such that the slit gap expands much beyond 1/2 inch.
The reasons for the progressive expansion which takes place in the graphite sleeve, are not fully known but, we feel, are the result of a bi-metal strip-like response at high temperatures between the outer diameter boundary surface of the susceptor (which comprises, after some use in growing silicon crystals, for instance, an uneven lattice of silicon carbide and free silicon) and the graphite wall. Silicon vapor freed in this crystal growing process tends to be drawn into the pores of the graphite susceptors and to be deposited as an outer coating in the form of free silicon. At the same time, the inner deposition of silicon and the release of silicon monoxide vapor from the liner converts some of the boundary graphite to silicon carbides at the high temperatures involved in the process. When the enclosure is cooled to ambient room temperature, its propensity to return to its original set or shape has been affected and, with repeated heating and cooling, this spread of the susceptor is progressive.
The principal object of the invention is to provide a method of restoring such graphite enclosure shapes, which are quite expensive, and to do so in an economical manner. Surprisingly, it has been reported to us that a restored susceptor has a longer life, i.e., seven to eight runs, than originally. For some reason its progressive expansion, after restoration, is more gradual.